An EGR is conventionally provided to reduce NOx discharge from an internal combustion engine.
Further, an EGR gas control valve for controlling an EGR gas amount is conventionally provided in an EGR gas passage that connects an exhaust gas passage and an intake air passage to each other, and a valve opening amount of the EGR control valve is controlled by outputting an EGR control valve opening command signal to the EGR control valve to realize a target EGR valve opening determined in accordance with operating conditions of the internal combustion engine.
However, when a fault occurs in the EGR control valve such that an accurate target EGR valve opening is not obtained in response to the EGR control valve opening command signal, an exhaust gas purification performance and an output performance of the internal combustion engine deteriorate. Therefore, various proposals have been made in the background art in relation to techniques for diagnosing a fault in an EGR control valve accurately and precisely.
For example, Japanese Patent Application Publication No. H10-122058 (Patent Document 1) discloses a technique in which, when an EGR operation condition according to which a target opening of an EGR control valve varies by at least a predetermined amount is established, an actual opening detection unit detects an actual opening, which varies so as to follow variation in the target opening, from the start of the variation in the target opening, and a fault is determined to have occurred in a device including the EGR control valve when it is confirmed that the detected actual opening is not varying so as to follow the target opening.
Further, Japanese Patent Application Publication No. 2007-255251 (Patent Document 2) discloses a technique for a device including an EGR control valve having a valve shaft, a driving unit which is disposed on an extension line of the valve shaft and includes a reciprocating drive shaft that reciprocates in an axial direction, and a control unit, wherein the reciprocating drive shaft of the driving unit is configured to open the EGR control valve by pressing a tip end of a central shaft of the EGR control valve when the driving unit is operative. In this technique, a determination as to whether or not a fault has occurred in an EGR control valve is made by determining whether a duty ratio of a control signal issued to the driving unit from the control unit is within or beyond an allowable range.
Patent Document 1: Japanese Patent Application Publication No. H10-122058
Patent Document 2: Japanese Patent Application Publication No. 2007-255251
In many cases, a fault in a control valve is caused by an increase in sliding friction resistance in a valve driving unit. Valve sliding friction causes a stick slip phenomenon, in which catching and sliding occur repeatedly during opening variation, and so on, and a deviation is particularly likely to appear in response to minute opening variation. Therefore, to detect abnormal phenomena early, it is advantageous to focus on a following error occurring in response to minute opening variation.
However, Patent Documents 1 and 2 disclose techniques of determining an abnormality on the basis of a deviation from an actual opening and a duty ratio range of a driving control signal rather than techniques for detecting abnormal phenomena by focusing on the following error occurring in response to minute opening variation.
It is therefore necessary to develop a technique for determining a fault in an EGR control valve with improved accuracy and precision.